In a letter to the regulatory agency on July 12, the Iowa Republican and ranking member of the Senate Judiciary Committee asked NHTSA to examine the phenomenon of "tin whiskers"--crystalline structures of tin--that could theoretically cause the vehicles to suddenly take off, according to a report on CNN.com.

Grassley cited documentation provided by whistle-blowers about the investigation by NHTSA and NASA into the Toyota problem, which resulted in the recall of millions of vehicles worldwide.

One report stated: "Because proof that the (electronic throttle-control systems) caused the reported (unintended accelerations) was not found does not mean it could not occur."

NHTSA confirmed it had received the letter and was reviewing it, but didn't have an immediate comment.

Grassley asked for NHTSA to provide his office with a response by July 26.

The information provided to his officer raises concerns about whether regulators' investigation was "too narrow" in scope, Grassley wrote.

He added that it also leaves unanswered questions about the causes of unintended acceleration, just as NHTSA moves to require brake-override throttles in response.

10-month investigation

A 10-month investigation by NASA and NHTSA into the matter found no electronic defect in Toyota models. The agencies blamed the incidents on pedals that got stuck or caught under floor mats.

But Grassley, in his letter, noted that the NASA report also stated: "Because proof that the (electronic throttle-control systems) caused the reported (unintended accelerations) was not found does not mean it could not occur."

NASA also reported finding tin whiskers in its analysis of the brake assembly, noting that they could cause a short circuit between the pedal signal and outputs. This condition could theoretically caused a car to unintentionally accelerate.

CNN.com quoted a Toyota spokesman who said that "so-called 'tin whiskers' are not a new phenomenon and do not represent a mysterious or undetectable problem in a vehicle's electronics."

A Toyota spokesman could not be immediately reached for further comment Friday morning.

In its review, NASA concluded that there were 9,698 identified unintentionally acceleration complaints from 2000 to 2010, but NHTSA concedes the number could be much higher because not all incidents are reported.

Toyota ended up paying fines of $48.8 million for failing to order the recall in a timely manner.

More aggressive policies

Since the Toyota recall, regulators have become more aggressive about addressing the issue. NHTSA has been exploring a brake-throttle override system mandate since 2010.

In April, the regulatory agency proposed a requirement that all light vehicles have an override mechanism that enables drivers to stop a car or truck, if the accelerator pedal gets stuck.

NHTSA didn't didn't offer an estimate on how much it would cost manufacturers to add the technology, only saying it could be done "without significant difficulty or cost."